Be suspicious (5,1,3)
I believe the answer is:
smell a rat
'be suspicious' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smell a rat that I've seen before include "Find something fishy" , "Become suspicious" , "Suspect something's amiss" , "Sense something suspicious" , "Suspect trickery (colloq.)" .)